The extracellular domain of a receptor binds with a ligand (L) on the outside of the cell. This causes a conformational change transmitted through the transmembrane portion of the protein, resulting in activation of the intracellular domain which then talks to other cytoplasmic proteins, causing a signalling cascade event. In some receptors, binding a ligand causes them to associate with adjacent receptors on the outside. This causes the inside domains to also cluster, which is then detected during a signalling event.